The Global Positioning System (“GPS”) is a global navigation satellite system typically used, as its name suggests, for navigation and guidance purposes. GPS is widely used for map-making, land surveying, commerce, scientific applications, and vehicle tracking and surveillance. GPS employs a constellation of between 24 and 32 Earth orbiting satellites, each of which transmits highly accurate radiowave signals. The radiowave signals allow a GPS receiver to determine its current location, its velocity, and the time. A GPS receiver calculates its position, including latitude, longitude, and elevation, by precisely timing the signals received from the GPS satellites. Once determined, the coordinates can be displayed on the GPS receiver as points.
Each of the GPS satellites transmits periodic messages containing the time the message was sent, precise orbital information, and the general system health and rough orbits of all GPS satellites. A GPS receiver determines the transit time of each received message and uses the transit time to compute the distance between the receiver and the corresponding GPS satellite. Geometric trilateration is used to combine the computed distances with the location of the satellites to calculate the GPS receiver's location. Additionally, time-based information, such as the direction and speed of the GPS receiver, can be calculated from a series of determined locations. GPS receivers also can store data indicating which satellites transmitted signals that were used, the potential precision of the data received, and one or more manually entered waypoints.
A track log (or “track”) is a collection of track points along the course of a route. A track log can be created by a GPS receiver as the user navigates a route. Further, a track log can be saved in a track file that lists one or more sequential coordinates in 3-D space, such as latitude, longitude and elevation. Additionally, a track file can be stored in a variety of file formats, such as the GPS Exchange Format (“GPX”) and the National Marine Electronic Association (“NMEA”) specification.